2 Answers
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Large icons are better, usually, because they make it easier for the user to operate the application. That being said, customizability is a nice goal, so that advanced users can reduce the size of the icons (or use keyboard shortcuts) to have a bigger view of the data.

Generally icons range from 16x16 to 32x32 to 48x48. Also bear in mind that the relative size of the icons should establish the relative importance of the functionality, so even if you increase the size of all your icons, ensure that the most important functions are given prominence via a larger size (for reference, see the 'Ribbon' toolbar in MS Office 2007/2010).

I don't know if this applies to your application, but you may need to consider that someone who is totally blind might be using the app with a screen reader. In that case, it's more important to include text labels than tweak the size of an icon that may never be seen at all.

There are different forms of visual impairment, as well. I think designers are giving more attention to potentially color-blind users, but you should remember that someone may be perfectly able to see detail but needs an interface with higher contrast or reversed colors.